How to use the testpath.commands.MockCommand function in testpath

def assert_calls(cmd, args=None):
    """Assert that a block of code runs the given command.
    
    If args is passed, also check that it was called at least once with the
    given arguments (not including the command name).
    
    Use as a context manager, e.g.::
    
        with assert_calls('git'):
            some_function_wrapping_git()
            
        with assert_calls('git', ['add', myfile]):
            some_other_function()
    """
    with MockCommand(cmd) as mc:
        yield
    
    calls = mc.get_calls()
    assert calls != [], "Command %r was not called" % cmd

    if args is not None:
        if not any(args == c['argv'][1:] for c in calls):
            msg = ["Command %r was not called with specified args (%r)" %
                            (cmd, args),
                   "It was called with these arguments: "]
            for c in calls:
                msg.append('  %r' % c['argv'][1:])
            raise AssertionError('\n'.join(msg))